It is generally believed that the engine equipped with a variable compression ratio (VCR) control system should work best if operated with a lower compression ratio under higher load conditions, and with a higher compression ratio under relatively lower load conditions. The higher load conditions occur, for example, at the start from a stopped state, during climbing on a hill and during an overtaking maneuver, and the relatively lower load conditions occur, for example, during cruising on a highway.
The VCR control system should be able to respond quickly enough to cope with any changes in load conditions that occur during a trip. The quick response of the VCR control system is necessary not only to assure smooth operation of the vehicle during the change in compression ratio takes place but also to protect the engine. For example, in a four-cylinder engine, the response and execution time of the VCR control system will have to be as short as that spent in one half rotation of the driveshaft to prevent knocking in the next combustion phase in the same cylinder.
It is probably true that the super-fast responding VCR control system such as described above should generally be able to operate the engine with a higher compression ratio than the compression ratio in a slow-responding VCR control system because the potential damage from operating the engine with a higher-than-ordinary compression ratio is much smaller than operating the engine with a slow responding VCR system, and/or that the VCR control system does not have to keep changing the compression ratio continuously as long as the compression ratio is kept in a permissible range and the system is ready to respond to any sudden large changes in all load conditions.
Our review of reports available in the public domain suggests that, in general, the hydraulic actuator tends to have a leak problem when operated under high pressure, and the mechanical actuator powered by the electric motor tends to have a longer response and execution time than desired.